A brushless motor generally has a permanent magnet as a rotor, and a current according to the position of the permanent magnet is applied to a coil as a component of the brushless motor, by using a power conversion circuit such as an inverter, thereby driving the brushless motor. A motor driving apparatus for driving such a brushless motor generally includes a power conversion circuit such as an inverter, and a control unit for controlling the power conversion circuit on the basis of the position of the rotor.
As a method for detecting the position of the rotor that is a permanent magnet, a position sensor such as a Hall element may be used, or the position of the rotor may be estimated from a current and a voltage applied to the coil (refer to “Practical Handbook of Motor Technology” editors version (chief editor: Daiki Ebihara), The Nikkan Kogyo Shinbun, Ltd., Mar. 23, 2001, pp. 145˜153).
As a method for controlling the rpm of the brushless motor, generally, the rpm of the rotor is calculated from rotor position information which is obtained by the above-mentioned position detection method, and a voltage or a current to be applied to the coil is adjusted on the basis of the calculated rpm and a command rpm. Thereby, the brushless motor can be driven at an arbitrary rpm.
However, when the input voltage of the power conversion circuit such as an inverter pulses at a constant frequency, if the brushless motor is driven at an arbitrary rpm as described above, noise and vibration may occur, or a harmonic current appears at the input side of the power conversion circuit, at an rpm that satisfies specific conditions.
Especially when the capacitance of a capacitor or a reactor provided before the power conversion circuit is reduced, a frequency component of the current for driving the brushless motor appears at the power supply side. The reason is as follows. The capacitor or the reactor functions as a filter to block a high-frequency component of the motor driving current that affects the power supply, and the effect of the filter is undesirably reduced due to reduction in the capacitances of these components. When the harmonic component of the motor driving current which appears at the power supply side increases, the motor driving apparatus cannot satisfy the IEC (International Electrotechnical Commission) standards as international harmonic standards.